started animation render- how to stop it without losing what has been rendered so far?
I don't think it's possible, that's why using jpg instead of movies is convenient, you can stop and relaunch whenever you want. Also, as said by Thibsert, "if Blender or the computer crashes, you don't loose the rendered frames. And it allows multiple computers work on the same animation (using a shared folder and the "placeholders/don't owerwrite" settings)".